1. Field of the Invention
The present invention relates in general to a molded joint boot obtained by using a thermoplastic polyester elastomer (TPE) as the molding material and relates in particular to a molded joint boot which is both formed into a bellows-like shape by using a TPE molding material having a high modulus of elasticity, and which is ideally suited to uses such as an enclosure boot for the universal joints in the transaxle of a front-wheel-drive automobile.
2. Description of the Prior Art
Several processes have conventionally been employed for obtaining bellows-like moldings in which the component material is formed into a pattern resembling a series of waves with an associated repeated sequence of crests and troughs. Such processes have included two types of blow molding, distinguishable in part by the manner in which the cylindrical parison workpiece is obtained. The first blow-molding process operates with respect to a screw-extruded parison, while the second utilizes a parison which is itself injection molded. The provided parisons are then blow-inflated within a metal mold of specified form, the parisons being sufficiently thick to suitably withstand the mold-conformance stretchings inherent in the inflation process.
The bellows-like moldings obtained through such blow-molding processes are nevertheless very thin, with usual thickness of about 0.7 to about 1.8 mm. It has consequentially been considered difficult to employ the referenced TPE's for the formation of such moldings through the use of injection-molding processes alone. Thus widely utilized instead has been the above-mentioned blow-molding process in which injection molding is employed only intermediately to produce the parison workpiece.
With evaluative regard to the two subject blow-molding processes, the extruded-parison process is deficient in that even if suitable means are employed to controllably vary the thickness of the produced parison, stretch-induced irregularities in the comparative thicknesses of the crest and trough portions within bellows-like moldings still result. Techniques for overcoming this deficiency have not as yet been presented. With regard furthermore to the injected-parison process, it is similarly quite difficult to obtain uniform thicknesses, thereby disadvantageously giving rise during use to such deformational anomolies as buckling. In addition, neither the extruded-parison nor the injected-parison processes have typically been suseptable to improvements in dimensional accuracies for the interior diameters of the final molded products. In view of these disadvantages, it would be very desirable to be able to alternatively employ a totally injection-molded process, with its superior dimensional-tolerance capabilities.
However, just as it has previously been regarded as being very difficult to employ the referenced TPE's as thin-wall molding materials in totally injection-molded processes, so also have there been few other injectably-appropriate materials with the requisite optimum degree of fluidity needed to achieve very-thin final-product wall thicknesses of about 0.7 to about 1.8 mm.
Furthermore, because conventional embodiments of the subject bellows have typically been formed of materials which are relatively soft, the centrifugal forces generated during high-speed rotational use have caused the associated wall members to become greatly extended, thereby coming into contact with surrounding parts where frictional wear then occurs. Moreover, because the innate structural strength of the subject moldings is relatively low, there is a resultant tendency toward breakage after only a short period of use, due to material fatigue as accelerated by repeated expansion and contraction.